how can i view linux kernel source code?

I tried to see the linux kernel source code but i could not get to do do so.
Ideally it should be there in the /usr/src directory but its not there.
Can somebody suggest me where it should be and how can I get to know about it?
I am using fedora core-5 linux 2.6 kernel.
Ideally i think it should be given with the CD as per GPL terms and condition? right?
Being a starter i dont know how to play with the kernel? Please suggest, how can I get the linux kernel to start kernel module programming?